1. Field of the Invention
This invention relates to an ultrasonic transducer, and more particularly to an ultrasonic source which can form a fine ultrasonic beam over a comparatively extensive scope in a predetermined medium.
2. Description of the Prior Art
In an ultrasonic transducer (as to which it is generally well known that an ultrasonic source is used also as a receiver for receiving reflected waves) having hitherto been employed in, for example, a diagnostic ultrasound equipment, a fine beam with a beam width of about 1 - 3 mm need be obtained in a comparatively extensive scope of about 1 - 30 cm in the interior of the body, in other words, at a depth of about 1 - 30 cm. The reason therefor is that the resolution depends upon the beam width and that a smaller beam width can achieve a higher resolution. As an expedient, it has been proposed to use a ring type transducer for the ultrasonic source.
FIG. 1 shows a plan view of the ring type transducer. In the figure, numeral 1 designates the ring type transducer power, while numeral 2 denotes an electrode disposed in any desired place of the transducer proper 1. The transducer proper 1 employs, for example, lead titanate zirconate as a piezoelectric material. By sintering the material in a conventional way, the ring is shaped by way of example into a radius r of 10 cm and a width D of 1 mm.
Upon condition that the width (D) of the ring is sufficiently smaller than the diameter (2 r) of the transducer (for example, the magnitudes r and D are set at the above-mentioned values), all the wave fronts which arrive at points on the axis of the transducer from the ring-shaped transducer proper are in phase and hence, the wave fronts are intensified with one another by constructive interference. In contrast, at points other than those on the axis of the transducer, the arriving wave fronts have phases which are mutually different, and hence, they are canceled from one another by destructive interference. In consequence, the ring type transducer can form a fine ultrasonic beam along the axis thereof. Accordingly, the ultrasonic source which employs the ring type transducer is suitable for generating ultrasonic waves of small beam width. Thus, as regards the directivity, the ring type transducer is far more excellent than a prior-art transducer of the surface type (flat disk, curved or concave disk).
The ring type transducer, however, has the following disadvantages as compared with the surface type transducer and has scarcely been put into practical use up to now. (1) On account of attenuation resulting from to the spherical spread, the sensitivity is conspicuously insufficient at a large depth. (2) In case where ultrasonic waves are transmitted into a medium and where reflected waves are received, a paste is interposed between the transducer and the medium. When the contact of the transducer with the medium is poor on account of the existence of air bubbles etc. in the paste, the directivity of the beam is disturbed. (3) The amplitude of a side lobe is large.